The invention relates to a water source heat pump system and more particularly relates to an air conditioning unit in a heat pump system utilizing a heat exchanger of the type having a water contact coil within a housing, with an aquastat disposed within the water contact coil to monitor the temperature of water passing therethrough.
Water source heat pump systems are those in which heat is injected into or extracted from flowing water, and the heat thus transferred is utilized to cool or heat air. Generally, the air to be conditioned by a water source heat pump is confined to selected zones within an enclosed building, such as, for example, rooms in a building where the temperature in each room is to be individually controlled. Each zone or room, for example, may contain an air conditioning unit to communicate with the water in the water source heat pump system. In utilizing the water in this manner, some air conditioning units may be heating while other air conditioning units may be cooling. In many cases, the means for transferring heat from the flowing water to the air conditioning unit is a water contact coil within a housing having a refrigerant therein such as a tube-in-tube heat exchanger or a water coil in a shell housing type heat exchanger. For example, in a tube-in-tube heat exchanger water flows through one tube and a refrigerant flows in the other with the heat being transferred according to the requirements of the air conditioning unit. If it is desired to put an air conditioner on a heating cycle where it is necessary to extract heat from the circulating water in the tube-in-tube heat exchanger, the water in the tube-in-tube heat exchanger gives up heat to the refrigerant passing through the exchanger thereby reducing the temperature of the water passing therethrough. In some cases, it has been found that the refrigerant entering the tube-in-tube exchanger is at a temperature below the freezing point of water and reduces the temperature of the water in the tube-in-tube exchanger to the freezing point of water thereby stopping the flow of water through the exchanger and subsequently causing problems in the operation of the air conditioning unit. In order to overcome this problem of water freezing in the tube-in-tube exchanger, several arrangements have been proposed. None herebefore have, however, proved to completely alleviate this problem. For example, one means proposed to prevent the freeze-up problem is to install a refrigerant suction pressure and/or temperature control device in or on the refrigerant line downstream of the heat exchanger so that at a preselected pressure or temperature, the air conditioning unit shuts down. However, this has proved unsatisfactory in that the refrigerant pressure or temperature control devices also shut down the air conditioner even when water flow and temperature are within safe limits. This is because the means to monitor the water condition is indirect and subject to conditions other than the temperature of the water, such as cold ambient air. Other suggested means have been to attach a temperature sensing device to the outside wall of the water conduit downstream of the heat exchanger or to install a temperature sensing device in the water stream downstream of the heat exchanger. In either of these cases, if the water flow is stopped for any reason, water does not flow from the exchanger and the temperature within is not reflected in the temperature sensing device as it is dependent upon water flowing from the heat exchange unit.